Importing FBX into Blender 2.9 results in warped mesh
I had the same issue so I'll share what I've been able to find on it. Its caused by an error with the skeleton. If you delete it, the mesh should fix itself. Apparently blender just doesn't support importing skeletons/armatures from FBX, but third parties do.
There is a workaround - https://darisl.wordpress.com/about/importing-fbx-files-into-blender-with-full-armaturesweights/
And software you can purchase - https://blendermarket.com/products/better-fbx-importer--exporter
I have yet to verify either.